The present invention relates to anomaly detection and characterization in a propagative medium, and more particularly to a time domain reflectometry system that is used for locating irregularities in a signal transmission medium.
In telecommunications and network applications, transmitters and receivers are connected together via signal transmission cables, such as coaxial cables or optical fibers. Faults in these cables often result in undesired attenuation of signals transmitted over the cables resulting in lost information. Time domain reflectometers are used to test these cables to determine if they have irregularities, such as faults or other discontinuities, that would interfere with the transmission of information.
Time domain reflectometry is similar to radar. Pulses are transmitted into the medium and during the interval between pulses the return signal is examined for anomalies. When light propagates through an optical fiber, the fiber material scatters the light in a process known as "Rayleigh scattering". Some of the light is scattered back through the fiber to the transmitter. This light is referred to as "backscatter". The backscatter signal from a pulse launched into an optical fiber decreases exponentially with the rate of attenuation of the fiber. A discontinuity in the fiber where light is lost but no light is reflected, such as a splice, appears in the backscatter signal as an anomalous drop over a pulsewidth. A reflective event such as a mechanical connector causes a reflected image of the pulse to be added to the backscatter. Often an irregularity in the fiber is both reflective and shows a loss. Locating events with unacceptably large losses is of interest when determining fiber quality and fault location. From the known index of refraction of the fiber and a plot of return signal-versus-time, the location of an event in question can be determined relative to a known event such as the beginning of the fiber or a nearby connector or splice.
Typically, prior art optical time domain reflectometers, such as the OF235 Fiber Optical Time Domain Reflectometer manufactured by Tektronix, Inc. of Beaverton, Oreg. United States of America, sample the return signal at many, closely spaced points along the length of the fiber. The resulting sample data points are then displayed as an amplitude-versus-time plot, or waveform. Because backscatter is a weak signal, each data point in the waveform must be sampled many times and the results averaged to reduce noise to an acceptable level. Depending on the length of the fiber and the desired resolution of the display waveform, it may take a considerable amount of time, approximately ten minutes, to collect the waveform of data points. In addition, an operator must measure and interpret the displayed waveform to determine event location and amount of loss.
What is needed is a method of detecting and characterizing significant discontinuities in the backscatter signal, using a minimum number of data points with a minimum amount of averaging.